A three-way catalyst is well known in the art as an exhaust gas purifying catalyst, which is provided in an exhaust pipe of a gasoline engine. The three-way catalyst purifies NOx, HC and CO contained in exhaust gas through oxidation-deoxidation reaction. And efficiency of the catalyst for purifying the exhaust gas will be increased up to a maximum value, when air-fuel ratio of mixture (that is, a ratio of intake-air amount versus injected amount of fuel) to be supplied to the gasoline engine is around a theoretical air-fuel ratio (that is, a stoichiometric air-fuel ratio). In addition, the three-way catalyst traps excessive oxygen contained in the mixture (exhaust gas) after combustion, when the air-fuel ratio of the mixture becomes larger than the theoretical air-fuel ratio (that is, when the mixture becomes leaner). On the other hand, the three-way catalyst discharges such trapped oxygen, when the air-fuel ratio of the mixture becomes smaller than the theoretical air-fuel ratio (that is, when the mixture becomes richer). Namely, when the air-fuel ratio becomes larger (i.e. the mixture becomes leaner), the excessive oxygen is trapped by the three-way catalyst, so that NOx in the exhaust gas is reduced (deoxidized). On the other hand, when the air-fuel ratio becomes smaller (i.e. the mixture becomes richer), the oxygen trapped in the three-way catalyst is discharged into the exhaust gas, so that HC and CO are oxidized. As a result, NOx, HC and CO contained in the exhaust gas can be purified, even when the air-fuel ratio of the mixture is changed to a richer side or a leaner side from the theoretical air-fuel ratio depending on operating condition of the gasoline engine.
When the three-way catalyst is deteriorated, an amount of the excessive oxygen in the exhaust gas which will be trapped into the three-way catalyst (also referred to as a maximum oxygen trapping amount) is decreased. Then, exhaust gas purifying performance is decreased and emission characteristic may be correspondingly decreased. Therefore, according to a prior art, for example, as disclosed in Japanese Patent Publication No. H04-081540, it is determined that the maximum oxygen trapping amount of the three-way catalyst is excessively decreased and the three-way catalyst is diagnosed as being deteriorated, when a response delay time becomes shorter than a predetermined threshold value. The response delay time is defined as a time period from a time point, at which oxygen concentration detected by an oxygen sensor (an O2 sensor) provided at an upstream side of the three-way catalyst is changed from a rich side to a lean side (or from the lean side to the rich side), to another time point, at which oxygen concentration detected by another oxygen sensor provided at a downstream side of the three-way catalyst is changed from the rich side to the lean side (or from the lean side to the rich side). The above diagnosis is carried out by an in-vehicle malfunction diagnosing apparatus (which is also referred to as OBD, that is, On Board Diagnosis). When the three-way catalyst is diagnosed as being deteriorated, an engine checking lamp is turned on to inform a vehicle driver of an abnormal condition of the three-way catalyst.
When a response of the oxygen sensor provided at the downstream side of the three-way catalyst is deteriorated, or when a dead-time delay occurs, the above diagnosis may not be properly carried out. Namely, when the above situation occurs, the response delay time detected during the diagnosing process becomes longer. Then, the three-way catalyst may be, by mistake, diagnosed as being in order, even in spite that the maximum oxygen trapping amount of the three-way catalyst is excessively decreased as a result of the deterioration of the three-way catalyst. Therefore, the engine checking lamp is not turned on, the abnormal condition of the three-way catalyst is not informed to the driver, and thereby vehicle operation may be continued in a condition, in which emission characteristic is decreased. On the other hand, when the above predetermined threshold value is changed to a safer side by taking is the possible deterioration of the response and/or occurrence of the dead-time delay into consideration, in order that the emission characteristic may not be decreased, then the three-way catalyst may be misdiagnosed as being deteriorated even in the case that the three-way catalyst is in good order.
The above problem (misdiagnosis for the deterioration of the exhaust gas purifying catalyst) is commonly observed not only in the exhaust gas purifying catalyst having the three-way catalyst, but also in the other exhaust gas purifying catalyst, in which a gas sensor is provided at a downstream side of the exhaust gas purifying catalyst and deterioration of the catalyst is diagnosed based on output value of the gas sensor, when the response of the gas sensor is deteriorated or the dead-time delay occurs.